Traditional display panels, such as cathode ray tube (CRT) displays are being replaced by more advanced alternatives, such as liquid crystal displays (LCDs) and plasma displays. These newer display technologies offer various advantages over traditional CRT technology, including reduced weight; thinner profile; superior color, contrast and brightness in the resulting display; and reduced power consumption.
LCDs typically include an array of pixels arranged in front of a light source, lighting panel, or reflector. Each pixel includes a layer of liquid crystal material, and two filters, with one filter serving to polarize light horizontally and the other filter serving to polarize light vertically. A reflective LCD has a reflective layer (such as a mirror) behind the pixels and is lit by either a frontlight or ambient light. A transmissive LCD is lit by a backlight, in an arrangement where the pixels are arranged in front of a light source and light from the light source is transmitted through the pixels to the front of the LCD or the viewing area, resulting in a “lit” pixel. Light sources for backlighting may include electroluminescent panels/foils (ELs), cold cathode fluorescent lamps (CCFLs), hot cathode fluorescent lamps (HCFLs) External Electrode Fluorescent Lamps (EEFLs), traditional incandescent light bulbs, or light emitting diodes (LEDs). High intensity discharge lamps (HIDs) may also be used in certain backlighting applications.
When LEDs are utilized as light sources for transmissive LCD backlighting, they may be traditional or side view LEDs. Side view LEDs are also known as “side emitting LEDs” or “side looker LEDs.” A side view LED is a packaged LED that emits light parallel to the plane of the surface to which the package is mounted. Compared to traditional LED packages, side view LED packages are thinner and typically less expensive to manufacture. A typical side view LED package has dimensions of about 1.5 mm by about 0.5-0.7 mm, but such dimensions may vary. Owing at least in part to their small size, side view LED packages have limited current handling capabilities, thus limiting their emissions. As a result, a single side view LED package typically has less output capability than a traditional LED package. Whether accidentally or deliberately, practitioners often “overcurrent” side view LEDs to achieve greater output, which can lead to overheating and failure of a device incorporating such LEDs.
Current LED lighting techniques for transmissive LCDs include either direct backlighting or backlighting by edge illumination. While a backlight will light the pixels from the backside, that light may come from directly behind the center of the pixels (direct backlighting) or from behind the sides of the pixels (backlighting by edge illumination). While both techniques are recognized as backlighting, direct backlighting refers to illumination directed axially (centrally) through the backside of pixels. In contrast, edge illumination refers to an arrangement where the light source is located along an edge of a LCD system and lights the pixels from the side. Edge illumination may occur along the side edges of the LCD, the top edge, or the bottom edge. Backlighting of transmissive LCDs may utilize a waveguide to spread the light to an entire LCD panel, with such waveguide having a single entry for light and transmitting the light by internal reflection to be spread over a desired area.
A traditionally LED backlit LCD may include a layered system that includes an array of red, green and blue LEDs positioned on a panel or array of tiles. The panel or tiles include a reflective surface or layer arranged so that light is reflected in the desired direction toward a diffusion layer that diffuses the light provided from the LED backlight and reflects some of the light back toward the reflective surface or layer, thereby functioning to mix the light and improve the light uniformity (“recycling effect”). A brightness enhancement film and/or collimating layer over the reflective layer or surface and the diffusion layer provides light to the LCD panel layers in a more optimal fashion and also acts with the reflective layer or surface and the diffusion layer to enhance the recycling effect. Depending on the embodiment, additional layers or fewer layers can be utilized as is will be appreciated by one skilled in the art. Such a backlight system may also include thermal layers below the LEDs for thermal management. As such, as is understood by one of skill in the art, the LED backlight system provides uniform white light to the LCD panels, which typically include a shutter layer and red, green and blue color filters.
The above-described traditionally lit screens have several inherent limitations. In the context of direct backlighting with a LED, a traditional LED package emits light directly at (i.e., through) a LCD screen. In order to light the entire screen, the LED must operate at high power and be very thick if utilized without a waveguide. Accordingly, the thickness of the resulting LCD system (television, monitor, etc.) is also increased. Modern consumers, however, desire thinner profile systems with thicknesses as small as a few inches. Accordingly, thick LCD systems are not commercially desirable. Additionally, use of high powered traditional LED package without a waveguide may create a “headlight effect” also known as a “hot spot” on the panel, causing a detrimental lack of light uniformity.
With regard to edge illumination, in which a waveguide is used to spread the light emission of a side view LED package disposed on an edge of the LCD, the resulting screen size is limited. As indicated previously, light emission characteristics of side view LED packages are reduced in comparison to traditional LED packages, with such limitation due to their size and current handling capabilities. As a result of the lower light emission, and the single entry for light in the waveguide, one or more portions (i.e., typically the center) of a side view LED-lighted LCD screen may not be lit if the screen is relatively large in size. LCD screens illuminated with side view LED packages and having acceptably uniform illumination characteristics are currently limited to screens of about 12 to 14 inches (diagonal) in size.
Accordingly, a need exists in the art for backlit LCD displays and display systems that are light in weight, have a thin profile, that enable use of large screens, and that are uniformly lit over the entire display with superior color and contrast. These and other needs are addressed with devices and systems according to embodiments of the present invention.